Maintenance Costs. Start With. Not when it comes to your pumps! Photo 1: Extensive damage on subject pump

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1 Maintenance Costs Start With By Robert Aronen and George Dierssen, Industry Uptime, and Mary Hoang, PE, San Jose City Municipal Water System Supersize it? Not when it comes to your pumps! High maintenance costs and poor reliability are frequently the result of improper sizing and design selection of centrifugal pumps. Unfortu-nately, the bid process often drives end users to pick the pump with the lowest price, not necessarily the best fit for the intended service. This case history is the result of a city water district s program to decrease operating and maintenance costs by increasing equipment reliability. In the process, the significance of proper hydraulic sizing and selection of the appropriate pump design type has become very clear. Background The Northern California municipal water district program to increase reliability includes eliminating bad actor pumps. Photo 1: Extensive damage on subject pump With its repeat failures, that s a designation the subject pump had carried for several years. Water district personnel had been pointing to cavitation noise and a history of poor reliability with this pump since its installation in The pump s failures had resulted in extensive damage, including broken shafts, bearing damage and heavy pitting of the impeller and case (Photo 1). The pump had been repaired and/or rebuilt at least four times in three years, resulting in high maintenance costs. Several repairs were completed by the original manufacturer, which did not recommend any changes or improvements. Finally, the water district sent the pump to Conhagen of California, a local, independent pump repair shop. Conhagen recognized the chronic failures and resultant damage as indicators of system or hydraulic problems (Photo 2), not one-time mechanical events. When the parties began discussing potential systems issues, IndustryUptime, a company specializing in pump reliability, ( com) was retained to perform PUMPS & SYSTEMS APRIL

2 vibration analysis and pump performance testing to determine the root cause of the poor reliability. Analysis and Testing Photo 2: Type of damage indicated system or hydraulic problems. Figure 1. Pump curve diagram for subject pump During review of the installation, the running pump emitted a loud cavitation-type noise. Cavitation in the traditional sense (lack of suction pressure), however, was ruled out, as the available suction pressure was well above that required. The pump curve indicated the NPSHR (Net Positive Suction Head Required) at flow was eight feet and the pump was running with 97 feet of NPSHA (Net Positive Suction Head Available). The suction piping was sized and run properly, flow velocities were acceptable, and a brief review of the system design ruled out potential air entrainment or vortexing. Therefore, the consultants concluded the problem was not associated with the suction system. To better understand the system, flow, pressure and vibration data were taken at minimum, maximum and intermediate operating points. Vibration data was recorded using a portable vibration analyzer. Pump flow rates were observed using on-site flow meters and pressure gauges. (This data could have been obtained using portable instruments as well.) Flow and pressure data were plotted on the pump curve to determine the operating range of the pump, as well as to verify that the pump was operating on its curve (see Figure 1). The operating range of the pump was between 450 and 900 GPM, and all points were on the curve, indicating that the pump was operating per design. A typical industry rule of thumb suggesting that minimum flow should be at least 50 percent of BEP would have indicated that most of this operating range was acceptable. Yet, cavitation noise was still audible at 900 GPM, signifying that the rule of thumb did not apply in this case. Identification and Explanation Upon plotting the flow and head data on the curve, it became apparent that the pump was operating well below the BEP (Best Efficiency Point) of 1250 GPM. Placing the operating data on the curve revealed that the pump was oversized for the application flow range, forcing it to run off design. Calculating the Suction Specific Speed showed that the pump design has a very narrow window in which it will operate with reasonable reliability. The combination of these two factors resulted in flow recirculation and poor reliability. Recirculation is a flow reversal phenomenon due to separation of the fluid at the suction and discharge tips of the impeller vanes. Recirculation causes pitting damage on the impellers and pump case (Photo 3). Similar to traditional cavitation, this type of damage will be to the pressure side of the impeller vanes, as opposed to the inlet side of the vanes. 1 To confirm that recirculation was, in fact, occurring, the pump Photo 3: Pitting damage caused by recirculation design was reviewed to determine acceptable operating flow range. The acceptable flow limits were determined using the Suction Specific Speed (S), a design measure which relates RPM, flow (Q), and NPSHR. S is a measure of how close to BEP flow, recirculation will begin. The equation 2 to determine S is: S = (RPM * Q 1 / 2) : NPSHR 3 /4 All values used in the calculation are for the maximum impeller diameter of a given design at BEP. The subject pump has a suction specific speed of 14,000, creating an extremely narrow acceptable operating range 26 APRIL PUMPS & SYSTEMS

3 from 1050 GPM to 1350 GPM (based upon Process Industry Practices; PIP RESP001). The pump operates between GPM. Therefore, it continuously operates outside the acceptable flow range, thus experiencing recirculation and the associated damage. Figure 1 shows the test points, operating range and acceptable operating range on the pump curve. An S of 14,000 is well outside standard industry practices for selecting reliable pump designs and is typically reserved for very specialized applications. The narrow flow range defined by this design would not satisfy system requirements even if the pump had been properly sized. A pump with a very high suction specific speed and single volute design is the typical result of a low bid process. Unfortunately, low bid generally delivers poor reliability unless appropriate purchasing requirements are in place. Requiring vendors to quote pumps with S < 11,000 is one simple method of making sure that marginal designs are not considered. While this should not be the only criterion, it is certainly an important one. Furthermore, whatever the value of S, the customer must evaluate each bid to ensure that the pump will not operate below the minimum continuous flow indicated by S Ḟor most pumps operating in process plants, S < 11,000 is considered acceptable. The higher the suction specific speed, the closer the pump must operate to BEP, and therefore, the more likely the pump is to experience recirculation 3. This is a widely used and accepted process industry practice to determine acceptable operating limits for centrifugal pumps. Higher S designs are the exception and typically reserved for high-energy and specialized designs. Pump designs with S > 11,000 should only be considered in applications where the control system is designed such that the pump will always run in the narrow operating window defined by such design. According to Process Industry Practices; PIP RESP001: Pumps with suction specific speed greater than 11,000 require specific approval by the purchaser. A quotation for such pump shall include minimum continuous flow rate, maximum operating flow rate and operating experience. Similarly API 610, Ninth Edition states: Pumps shall have a preferred operating region of 70% to 120% of best efficiency flowrate of the pump as furnished. The vibration data collected from the subject pump also showed evidence of hydraulic Circle 217 on Reader Service Card PUMPS & SYSTEMS APRIL

4 ed a high-level predominant peak at seven times operating speed corresponding to vane pass frequency for this pump. Vibration amplitude decreased by roughly 40 percent when flow rates increased. Cavitation noise also decreased at higher flows. Now that the cavitation issue was understood, why did In order to achieve low NPSHR - values, an end suction design incorporating an enclosed impeller with a large eye and injection bores is standard the shaft break? Low-flow operation generates large, unbalanced hydraulic forces that act upon the impeller and deflect the shaft. Excessive bending stresses are imposed upon the shaft at a high cycle rate (equal to RPM). Eventually, the shaft material becomes fatigued and the shaft breaks. Compounding the problem, the subject pump is a single-volute design, in which radial loads increase substantially at low flows (in contrast to dual volute designs wherein hydraulic forces are more balanced). Supporting mechanical evidence of the failure is two-fold. The as found section of the repair report states: the case rings have a groove worn in toward one side. This is an indicator that the pump impeller is running off center, against the stationary wear rings. The break in the shaft is of circumferential nature, as shown in the report photos. The shaft fractured at 90 to the shaft, which is indicative of a fatigue failure (as opposed to a 45 fracture failure, which is indicative of a torsional load-related failure). Circle 210 on Reader Service Card The Solution The water district considered several alternatives, including: 1. Operating existing pump within given design parameters of GPM. This would be accomplished by increasing the amount of water that is re-circulated back to the suction piping network. Changes to the operating parameters of pumps that operate sequentially (to cover the large flow range) would also have to be modified. Though this option would require little upfront expense, it would result in increased power costs on the order of $10,000 annually. 2. Installing a Variable Frequency Drive (VFD) to vary the speed of the motor. Though this option potentially allows for the greatest range in flow rates and reduction in power costs, it is the most expensive solution and requires a large initial capital investment in the installation and purchase of a VFD. To determine if this option would be worth exploring, a cost benefit analysis would need to be done by comparing the estimated annual energy savings over the 28 APRIL PUMPS & SYSTEMS

5 life of the pump. 3. Replacing the impeller with one suited for the head and flow range of operation. Although this conversion would cost approximately $8,000 in material and labor, it would increase the reliability of the pump and provide a larger acceptable operating window. In addition, the pump efficiency would increase, resulting in power savings of approximately $10,000 a year. Two main factors affected the water district s evaluation of these proposed solutions. First, the district would be changing operational parameters in six months due to increased demand. Second, in two years, the district would be constructing a reservoir to stabilize demand, at which time the subject pump wood likely be retired. Given the operational requirements and two-year project life, Option #3 installing a replacement impeller suited for the required head, flow and range of operation clearly provides the best solution. Fortunately and ironically, the replacement impeller is available through the original manufacturer. No pump case or other major modifications will be required, so such a conversion is relatively inexpensive and straight-forward. Once the new impeller is installed, the acceptable window of operation will range from 500 to 1125 GPM. The new impeller fulfills the original design requirements and is within industry guidelines for reliable pump design. The BEP flow will be reduced to 900 GPM and suction specific speed to System settings will be adjusted to ensure minimum continuous flow requirements are met. Operating under the new conditions, the pump will be reliable and have a larger acceptable operating window. In addition, the pump efficiency will increase, resulting in a power savings on the order of $10,000 per year. Conclusions The costly maintenance problems associated with the subject pump resulted from the initial sizing and design selection. Oversized for the application flow range, the pump has a very narrow window in which it will operate with reasonable reliability. The conditions that led to oversizing in this case are unknown. However, in determining the appropriate hydraulic sizing of a centrifugal pump, it is imperative that accurate system Circle 544 on Reader Service Card PUMPS & SYSTEMS APRIL

6 Circle 397 on Reader Service Card parameters be defined. Wellintentioned personnel must avoid the trap of adding excess capacity just to be on the safe side. Process designers, engineers, suppliers and manufacturers frequently add contingency capacity to ensure that the pump will not be undersized. The result is an oversized unit. A large percentage of problem pumps are simply oversized leading to unreliable, and inefficient operation. To ensure reliable bids and that their pumps are not needlessly oversized end users should furnish a detailed process specification, then confirm that suppliers are quoting a good fit for the service. The process specification should provide normal operating flow, minimum and maximum flow and the NPSHA, plus the head required at maximum operating flow. In this case, the original specification showed a large potential flow range. Given this requirement, the supplier should not have proposed a pump with S = 14,000, and could have provided a reliable energy-efficient pump for the same price. To ensure this situation is not repeated, the water district is updating its pump standard to require S < 11,000. The new purchasing standard will ensure that future pumps the district buys have operating windows that fit a reasonable level of system design. Ultimately, to ensure quality, cost-effective pump selections, users should adopt a life cycle cost approach to bid review. Many companies have developed models to estimate the total cost of ownership for a pump, taking into consideration purchase price, installation cost, operating efficiency and projected reliability. More information is available from the Hydraulic Institute website at where there are a number of resources related to pump selection, including the handbook, Pump Life Cycle Costs: A Guide to LCC Analysis for Pumping Systems. P&S 30 APRIL PUMPS & SYSTEMS

7 Bibliography 1. Karassik, I. J.; Messina, J. P.; Cooper,P.; Heald, C. C., The Pump Handbook, Third Edition, McGraw- Hill, 2001, p Ingersoll-Rand, Cameron Hydraulic Data, Seventeenth Edition, Ingersoll-Rand, Woodcliff Lake, NJ, 1988, p Karassik, I. J.; Messina, J. P.; Cooper, P.; Heald, C. C., The Pump Handbook, Third Edition, McGraw- Hill, 2001, p About the Authors Robert Aronen has 12 years of experience in improving pump reliability, including time spent working for several refineries and a mechanical seal manufacturer. Since co-founding IndustryUptime in 2000, he has helped implement more than $2 million in pump reliability improvements. He also provides rotating-equipment applications support to Dupont Corporation for its non-metallic material for pump wear components. Aronen received his B.S.M.E. from Rose-Hulman Institute of Technology. Contact him directly at raronen@industryuptime.com George Dierssen has been working with rotating equipment for more than 20 years. His comprehensive knowledge of machinery systems stems from having spent six years with a large pump OEM and eight years as a Project Engineer and Senior Reliability Engineer at two California refineries. Dierssen has an extensive background in improving pump reliability and efficiency, as well as in meeting modern emissions requirements with mechanical seals. He received his B.S.M.E. from California State University, Sacramento. Contact him directly at: gdierssen@industryuptime.com Mary Hoang, PE, is the Operations and Maintenance Manager for the San Jose City Municipal Water System. She has worked for the City of San Jose, CA, for 14 years and the Municipal Water System for eight years. A registered Professional Civil Engineer, Hoang earned both her B.S. in Civil Engineering and her M.A. in Public Administration from San Jose State University. Circle 232 on Reader Service Card PUMPS & SYSTEMS APRIL